In U.S. Pat. No. 2,905,601, an electrolytic bath for plating gold or a gold alloy is disclosed which contains a cyanide of gold, a base metal salt such as of cobalt, nickel, indium, etc., and citric acid plus sodium citrate or aceticacid plus sodium acetate. The combination of, e.g. citric acid and its salt is clearly intended to act as a buffer to maintain the bath within a pH range of about 3-5. The use of other weak acids such as lactic, formic, etc., is mentioned, but there is no demonstration of a bath containing formic acid. A current density range of 1-100 ASF (amperes per square foot) with only 10 ASF being demonstrated, and a temperature range of 50.degree. to 120.degree. F. with 70.degree. F. being preferred, are disclosed. U.S. Pat. No. 3,104,212 differs from the above in that the base metal salt is omitted.
U.S. Pat. No. 3,672,969 discloses a gold plating bath which contains an organophosphorus chelating compound, typically a phosphorus acid, e.g. amino-tri (methylphosphonic acid) or 1-hydroxyethylidene-1,1-diphosphonic acid. As an improvement, a water soluble citrate is included in the bath. However, there is no mention of formic acid.
The production of gold-copper-antimony alloys is discussed in U.S. Pat. Nos. 3,380,814 and 3,380,898. A complexing agent such as ethylenediaminetetraacetic acid (EDTA) is employed in the bath and a weak acid and salt thereof to provide a pH of 4.5 to 6.0, exemplified KH.sub.2 PO.sub.4 (a partially neutralized acid salt), partially neutralized citric acid, tartaric acid or acetic acid.
The use of nickel or cobalt chelates as brightener/hardeners is taught in U.S. Pat. Nos. 3,149,057 and '058. The use of aliphatic acids of 2 to 8 carbon atoms such as acetic, citric, tartaric, etc., when properly neutralized to act as buffers to maintain a pH between 3 and 5, is described. U.S. Pat. No. 4,186,064, discloses phosphate salts and citric acid salts as the conducting and buffering agents, and cobalt or nickel chelates of an organophosphorus compound such as nitrilotri (methylene phosphonic acid).
U.S. Pat. No. 4,253,920 discloses a gold plating bath which includes potassium dihydrogen phosphate, a Cu or Ni hardener/brightener and, as chelating agent, 1-hydroxyethylidene-1, 1-diphosphonic acid. No weak organic acids are present. In U.S. Pat. No. 4,197,172, the chelating agent is nitrilotris (methylene) triphosphonic acid (sold as Dequest 2000). U.S. Pat. No. 4,396,471 states that virtually any conductive acid or salt may be used as electrolyte and the composition of the electrolyte is not critical, mentioning weak organic acids such as malic, formic, and especially citric. Potassium citrate plus citric acid to buffer the bath, is recommended.
Commercially, parts to be plated can be plated on a continuous basis on reel-to-reel selective plating machines, see "Continuous Reel-to-Reel Plating for the Electronics Industry" by Jean Lochet et al, an AES Electronics Lecture. Such machines are very expensive and perform all the plating steps on a continuous basis, including cleaning, activation, undercoating, and final plating of the parts by processing the parts, in successive steps, through the complete plating cycle. Basically, their processing speed is only limited by the deposition speed, i.e. the ability of the plating baths to produce acceptable deposits of required thicknesses rapidly. It can be seen as a matter of economics that high deposition rates are highly desirable, since the higher the production is, the lower the unit cost becomes.
The introduction of continuous selective high speed plating required gold solutions capable of plating at much higher speed and current densities. At first, when low gold prices prevailed, this was met simply by increasing the gold concentration of the bath, because as a general rule higher gold concentrations permit higher efficiency, current densities and plating rates. That is, in the typical gold bath of U.S. Pat. No. 2,905,601 with 8 grams per liter of gold, this was increased to 32 grams per liter and even higher to obtain higher current density and plating speed. However, with the advent of greatly increased gold prices, this became impractical. For economic reasons (lower inventory, lower drag out, etc.) gold contents should be kept as low as possible. Consequently, other routes were sought to obtaining high speed gold plating baths with lower gold concentrations and high acceptable current densities.
Formulations were proposed making use of so-called current extenders. Typically, such current extenders increase the bath's ability to plate at high current densities without the deposit being burnt. A burnt deposit is spongy and black. It will be understood that higher current densities mean higher rates of deposition, since theoretically one ampere will deposit a definite amount of metal in one second.
As illustrative, in U.S. Pat. No. 3,929,595, the current extender is a heterocylic azohydrocarbon sulfonic acid or salt thereof. In U.S. Pat. No. 4,436,595, glycolic acid with a salt thereof is used as current extender. However, the addition of heterocyclic azohydrocarbon sulfonic acids or salts thereof or of glycolic acid and its salts, to gold plating solutions, reduces significantly the current efficiency, expressed as mg/ ampere-minute, to very small values, rendering the buildup of the thick bright deposits difficult or impossible in high speed applications in which thick deposits have to be built up in a very short time, termed "retention time". That is, the low current efficiency works oppositely to the effect of high current density. The low efficiency of these baths at high current densities could be overcome by increasing the temperature from the usual maximum range of 120.degree.-130.degree. F. to 150.degree. F. However, when that is done, the resulting deposits become dull or even burnt, hence unacceptable. Thus, such current extenders, although improvements for certain applications, are of limited interest or impractical for some high speed applications. As stated in U.S. Pat. No. 4,436,595 at column 3, lines 25-29, the lower the temperature, the brighter the deposit, but the slower the plating speed, and vice versa; and as a compromise between brightness and plating speed, an operating temperature of 130.degree. F. is preferred. In fact, in practice, very few if any known acid gold plating baths give bright deposits at 150.degree. F., whereas, as will be seen in the ensuing description, the reverse is true for the baths of the present invention.
Furthermore, in many instances, the deposits produced by some high speed electrolytes still fall short of expectation for the following reasons:
High current density plating in the order of 500 to 1000 ASF at the cathode results in similar, and in some cases because of very small anode areas, in even higher anodic current densities. Such high anodic current densities are highly undesirable because of anodic oxidation phenomena.
In most cases, the cobalt and/or nickel brighteners/hardeners usually present in the valency of 2 are oxidized to the higher valency of 3 and/or even changed to the highly undesirable inactive potassium cobalticyanide K.sub.3 [Co(CN).sub.6 ] or similar hydroxy complexes of the same family. The gold is also, in some cases, partially or even fully oxidized to the higher valency of 3, hence considerably reducing the efficiency and the rate of plating. Also, oxygen is often absorbed by the electrolyte and decreases efficiency and worsens metal distribution, as discussed in U.S. Pat. No. 3,669,852 recommending several methods to remove oxygen from gold plating baths.
In U.S. Pat. No. 3,475,290 an alkyl or alkylene quanidine compound is used in the bath and a large quantity of reducing agent such as formic acid is used to prevent its decomposition.
U.S. Pat. No. 3,904,493 discloses gold sulfite plating baths containing organophosphorus compounds such as phosphonic acids. A brightening agent such as nickel may be included in the baths. The addition of mineral or organic acids, bases or buffers to control pH, within a range of 5 to 11, is mentioned but the choice is not critical. Current densities useful for the baths are rather limited, e.g., of the order of about 1 A/dm.sup.2.
Other disclosures of general interest in this area are:
U.S. Pat. No. 3,893,896; PA1 U.S. Pat. No. 4,075,065; PA1 U.S. Pat. No. 4,076,598;
U.K. patent application No. 2,039,532A;
"Selective Plating Equipment--What Are the Options?," by Douglas R. Stewart, AES Symposium on Economic Use of and Substitution for Precious Metals in the Electronics Industry, Sept. 16-17, 1980; "Multi-Lateral Thicknesses in Individually Plated Stripes", by Brian C. Dowling, Second AES Symposium, Oct. 5-6, 1982; "Super Selective Reel to Reel Plating", by Peter Meuldijk, Second AES Symposium, Oct. 5-6, 1982; and Products Finishing, pp. 21-22, 24-25, January, 1941.